The Collaborative International Dictionary of English v.0.48:

Disproof \Dis*proof"\, n. [Pref. dis- + proof. Cf. Disprove.]
   A proving to be false or erroneous; confutation; refutation;
   as, to offer evidence in disproof of a statement.
   [1913 Webster]

         I need not offer anything farther in support of one, or
         in disproof of the other.                --Rogers.
   [1913 Webster]

WordNet (r) 3.0 (2006):

disproof
    n 1: any evidence that helps to establish the falsity of
         something [syn: disproof, falsification, refutation]
    2: the act of determining that something is false [syn:
       falsification, falsifying, disproof, refutation,
       refutal]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "disproof":
   abjuration, abjurement, abrogation, absolute contradiction,
   annulment, contradiction, contrary assertion, contravention,
   controversion, countering, crossing, denial, disaffirmation,
   disallowance, disavowal, disclaimer, disclamation, disownment,
   forswearing, gainsaying, impugnment, nullification, recantation,
   refutation, renunciation, repudiation, retractation, retraction,
   revocation